Why are the cities of Reno and Sparks considered separate cities?

420,000 is the combined population and I think they would attract more tourists and have downtown see lots more buildings being built. I also read the the general area of Truckee Meadows (Verdi, Reno, Sparks, Fernley, Fallon) has a combined population of 600,000 people, by passing Vegas. Why don't they just call the entire place Reno and get it over with?